Charging system for electrostatic generators



Aug. 7, 1962 R. w. CLOUD 3,048,720

CHARGING SYSTEM FOR ELECTROSTATIC GENERATORS Filed Sept. 15, 1958 United states Patent 3,048,720 CHARGING SYSTEM FOR ELECTROSTATIC GENERATORS Robert W. Cloud, Lexington, Mass, assignor to High Voltage Engineering Corporation, Burlington, Mass., a corporation of Massachusetts Filed Sept. 15, 1958, Ser. No. 761,255 3 Claims. (Cl. 310-) This invention relates to electrostatic belt-type generators and in particular to a novel method of and apparatus for the transfer of electric charge between the charge carrier and the controlled potential portions of the apparatus; namely, the high voltage terminal and ground.

In general, charging mechanisms for electrostatic belttype generators are of two types: the corona discharge method of charge transfer and induction-conduction charging methods. The present invention is concerned with the latter of the two methods. Various proposals have been made for induction-conduction charging methods. In one, as disclosed in U.S. Patent No. 2,697,793 to Trump and Cloud, the belt itself consists of a semi-conductive material and is charged by induction-conduction methods. In another type, suggested by Raymond G. Herb at the University of Wisconsin, the necessary conducting zones may be introduced into the belt very simply by using staples.

Induction-conduction charging has definite advantages over corona charging. With induction-conduction charging, there is no ionization of the insulating gas by the corona discharge wtih resultant production of corrosive lay-products. In corona charging the production of these by-products is undesirable in any event, and even prevents use of certain gases which would be good insulators such as sulphur hexafluoride. In the present state of the art, semiconductors have not been developed to a sufiicient degree to permit use of the device disclosed in the aforementioned U.S. Patent No. 2,697,793. The use of staples in the belt has been used successfully, but the current is less than that obtainable by corona methods. In accordance with the invention, it is possible to get charging currents comparable to those available by corona, and various other advantages are attendant upon the present invention, as will appear from the following description.

The invention may best be understood from the following detailed description thereof having reference to the accompanying drawing in which:

FIG. 1 is a diagrammatic view of an electrostatic belttype generator embodying the present invention;

'FIG. 2 is a detail in perspective showing the charging mechanism of the device shown in FIG. 1;

FIG. 3 is a front elevation showing an alternative charging mechanism constructed in accordance with the invention;

FIG. 4 is a front elevation showing an alternative method of charging in accordance with the invention; and

FIG. 5 is a side elevation of the apparatus shown in FIG. 4.

Referring to the drawing and first to FIGS. 1 and 2 thereof, the operation of the electrostatic belt-type generator therein shown is in general well known and need not be described herein in any detail. Suffice it to say that electric charge is carried by an insulating belt 1 supporting between two pulleys 2., 3 from the grounded end of the apparatus to a hollow electrode 4 which constitutes a high voltage terminal. Details of operation of electrostatic belt-type generators may be found, for example, in United States Patent No. 1,991,236 to Van de Graff and No. 2,252,668 to Trump and at vol. XI, page 1 of Reports on Progress in Physics (1948). In accordance with conventional induction-conduction charging as shown in U.S. Patent No. 2,697,793, an inducing plate is provided near the pulley so that the electric field is created from the pulley to the plate. Electric charge is then transferred between the pulley and the conductive parts of the belt which come in contact with the pulley. In accordance with the present invention, two inducing plates 5, 6 are provided so as to flank the belt 1 and a contacting leaf 7 is provided so that the conductive portions 8 of the belt 1 are connected to a fixed potential as they travel between the inducer plates 5, 6. The inducing plates 5, 6 are connected together by a lead 9 so as to be at a common potential which differs from that of the leaf 7. As a result, there are two electric fields, one on each side of the belt 1. By using two fields in this manner, it is possible to put as much charge on the belt as the belt is capable of carrying to the terminal, since both the charge-carrying capabilities of the belt as well as the rate of charging are limited by the breakdown gradient in the insulating gas. Hence by using two fields in accordance with the invention, the charging capabilities are doubled over that obtainable with a single field as applied at the pulley. The potential difference between the contact and the inducing plates may be created in any conventional manner, and in FIG. 1, the leaf 7 is shown as being grounded, while the inducing plates 5, 6 are maintained at a positive potential by a voltage source 10. As a result, negative charge is deposited on the belt 1.

In addition to moving the charge transfer location from the pulley and using two fields rather than one, my invention also comprehends imbedding the charge-carrying Wires or other conductors in the middle of the belt. In the case of induction-conduction charging at the pulley, this is impossible since the conducting portions must come in contact with the pulley. The conductive material in the middle of the belt may be wire or conducting rubber and may conveniently be positioned by sandwiching the wire or conducting rubber between two pieces of belt vulcanized together. In the embodiment shown in FIG. 2, the wires 8 stick out at one end of the belt 1 but not at the other, and electrical contact is made with the leaf 7 via these projecting ends 11. In FIG. 2, the extent of projection is somewhat exaggerated for clarity; the wires 8 need only project just beyond the surface of the belt 1. One of the main advantages of imbedding the conductors in the solid dielectric of which the belt is made is the resultant higher breakdown gradients which permit more charge to be carried by the belt.

Referring now to FIG. 3, the belt 1 therein shown is constructed in the same manner as the belt 1 shown in FIG. 2, except that the leaf contact 7 of FIG. 2 is shown in FIG. 3 as being replaced by a pulley 12 in order to reduce friction.

In the embodiment shown in FIGS. 4 and 5 both ends of the wires 8 are buried in the dielectric of the belt 1 in order to prevent corona which might otherwise result from the relatively sharp ends of the wire 8. Contact is made at the center of the belt 1 between an exposed area as shown at 13 and a pulley 14 or a leaf as hereinbefore described. In order to prevent any mechanical pressure on the belt 1, a pair of pulleys 14, 15 may be provided as shown in FIG. 5 in order to compensate for lateral mechanical forces.

In prior art devices wherein induction-conduction charging is done at the pulley, the current in a conventional generator having a belt five inches in width was limited to 200 microamperes. Using two inducing fields in accordance with the invention described herein, but without imbedding the conductors in the belt, current available was increased to 350 microamperes. With proper design of the belt in accordance with the invention currents of 500 microamperes may be obtained on a five-inch belt.

The charge may be removed in the high voltage terminal 4 by merely contacting the exposed portions of the conductors 8 in the belt 1, as by a simple brush 15. If it is desired to charge the down run of the belt, the charge transfer mechanism within the high voltage terminal may be identical to that described herein with appropriate reversal of polarity.

Having thus described the principles of the invention together with several illustrative embodiments thereof, it is to be understood that although specific terms are employed, they are used in a generic and descriptive sense and not for purposes of limitation, the scope of the invention being set forth in the following claims.

I claim:

1. An electrostatic generator comprising in combination a hollow electrode and an insulating traveling belt having conductive zones of which the dimensions are short in the direction of travel of said belt but extending substantially the width of the belt transverse thereto insulated from one another along the length of said belt, charging means for producing a net charge on said conductive zones at the grounded end of said electrostatic generator, and means within said hollow electrode for transferring said charges from each conductive zone to said hollow electrode when such conductive zone is electrically completely enclosed by said hollow electrode, said charging means comprising a pair of inducing plates flanking said belt and a contact maintained at a fixed potential with respect to said inducing plates and adapted to make temporary electrical contact with said conducting zones at their exposed positions during travel of said conducting zones between said inducing plates.

2. An electrostatic generator comprising in combination a hollow electrode and an insulating traveling belt having conductive zones of which the dimensions are short in the direction of travel of said belt but extending substantially the width of the belt transverse thereto insulated from one another along the length of said belt and imbedded therein so that only a small portion thereof extends beyond the surface of said belt, charging means for producing a net charge on said conductive zones at the grounded end of said electrostatic generator, and means within said hollow electrode for transferring said charges from each conductive zone to said hollow electrode when such conductive zone is electrically completely enclosed by said hollow electrode, said charging means comprising a pair of inducing plates flanking said belt and a contact maintained at a fixed potential with respect to said inducing plates and adapted to make temporary electrical contact with said conducting zones at their exposed positions during travel of said conducting zones between said inducing plates.

3. An electrostatic generator comprising in combination a hollow electrode and an insulating traveling belt having conductive zones of which the dimensions are short in the direction of travel of said belt but extending substantially the width of the belt transverse thereto insulated from one another along the length of said belt and imbedded therein, said belt being longitudinally divided so that only a small portion of each zone between the lateral extremities thereof is exposed, charging means for producing a net charge on said conductive zones at the grounded end of said electrostatic generator, and means within said hollow electrode for transferring said charges from each conductive zone to said hollow electrode when such conductive zone is electrically completely enclosed by said hollow electrode, said charging means comprising a pair of inducing plates flanking said belt and a contact maintained at a fixed potential with respect to said inducing plates and adapted to make temporary electrical contact with said conducting zones at their exposed positions during travel of said conducting zones between said inducing plates.

References Cited in the file of this patent UNITED STATES PATENTS 2,610,994 Bosch et al. Sept. 16, 1952 2,644,903 Lehrer July 7, 1953 2,697,793 Trump et al. Dec. 21, 1954 

